1. Field of the Invention
The present invention relates to a charging circuit, and more particularly, to a charging circuit which can adjust a charging period and a charging slope and provide a soft start operation.
2. Description of the Prior Art
The prior art usually utilizes a voltage generator to process a charging operation or a soft start operation for a MOS transistor capacitor, a polysilicon capacitor or a passive circuit. Hereinafter, the MOS transistor capacitor is realized by the metal-oxide-semiconductor field-effect transistor (MOSEFT), which is formed from the top to the bottom as a metal layer, which can be replaced by the polycrystalline silicon nowadays, an oxide layer and a P/N-type semiconductor layer to structurally form a MOS-transistor-type capacitor. The oxide layer is the silicon dioxide to be regarded as a dielectric material in the capacitor. Width of the oxide layer and a dielectric constant of the silicon dioxide determine a capacitance of the capacitor. The polycrystalline silicon is utilized to be the gate and the semiconductor is utilized to be the base, which form two terminal ends of the MOS transistor capacitor.
Please refer to FIG. 1A, FIG. 1B and FIG. 2, wherein FIG. 1A illustrates a schematic diagram of a conventional MOS transistor capacitor MOS_C for a charging operation, FIG. 1B illustrates a schematic diagram of different operational conditions of the MOS transistor capacitor MOS_C versus different capacitances, and FIG. 2 illustrates a schematic diagram of a terminal voltage VC1 of the MOS capacitor MOS_C at different timings. As shown in FIG. 1A and FIG. 1B, the MOS transistor capacitor MOS_C utilizes a stable current source CS for continuously charging operation. Due to an incremental voltage value of a gate of the MOS transistor capacitor MOS_C, the MOS transistor capacitor MOS_C changes capacitances thereof between a depletion capacitance C_del and an inversion capacitance C_inv, and the above capacitances correspond to different operational conditions. Please refer to FIG. 2, since the MOS transistor capacitor MOS_C can be either the depletion capacitance C_del or the inversion capacitance C_inv, the terminal voltage VC1 of the MOS transistor capacitor MOS_C corresponds to two lines with different slopes at the threshold voltage Vth, which equals to 0.8 volts. Under such circumstances, dramatically changeable capacitances of the capacitance of the MOS transistor capacitor MOS_C occur nearby the threshold voltage Vth.
Also, the prior art usually utilizes solutions, such as reducing charging currents of the MOS transistor capacitor MOS_C or increasing the capacitance of the MOS transistor capacitor MOS_C, to slow down the charging operation of the MOS transistor capacitor MOS_C to meet different requirements. However, the mentioned two solutions still have problems thereof. For example, if the charging currents are reduced, a leakage current can effectively influence the charging operation of the MOS transistor capacitor MOS_C. Besides, the incremental capacitance of the MOS transistor capacitor MOS_C may results in extra areas needed in circuit layout to increase product cost. Therefore, it has become an important issue to provide another charging circuit for the MOS transistor capacitor to prevent discontinuous charging voltages during the charging operation of the MOS transistor capacitor MOS_C, so as to adaptively provide an adjustable charging slope and an adjustable charging period to be operated as another soft start operation.